Nematodes sap Arkansas soybean yields

Twenty-five years ago, in one of the first cotton fields he worked as a researcher, Terry Kirkpatrick asked the grower if he’d ever sampled for nematodes. “He looked at me, serious as could be, and said, ‘I’m not growing soybeans. Why would I want to sample?’”

That illustrates, the Arkansas Extension plant pathologist said at the recent Arkansas Soybean Research Conference in Brinkley, Ark., “how well, even 25 years ago, the soybean farming community understood nematodes could affect their operations. The problem is they only understood soybean cyst nematodes.”

Unfortunately, as the region increasingly trades other crop acreage for soybeans, “we’ve kind of forgotten nematodes and it’s an emerging issue.”

The South “has a problem that the Midwest doesn’t. Several years ago, we shot our bullets in terms of effective variety resistance and now have very few effective varieties against soybean cyst nematode races.”

Another major issue Kirkpatrick sees is that the races of soybean cyst nematode today are much more subtle. Back in the days of Race 3, “it actually messed with the nodulation on the plants and they’d turn yellow. It was very easy to see. The races today don’t do that. They’re pretty easy on the plants in terms of visibility. Now, in very bad cases they’ll stunt the plants. But races 2, 5 and 6 are capable of taking 10 or 12 bushels out from under a farmer and he doesn’t even know there’s a problem.”

The biggest nematode issue over the last several years, however, is root-knot, said the pathologist. “I think that’s the case for several reasons. First, there’s just no highly effective resistance to root-knot. There are a few varieties that are (fractionally) less susceptible. But when talking about the majority of the soybean cultivars we’d grow in Arkansas, they’re susceptible to root-knot.”

The widespread adoption of early-maturing soybeans in the Mid-South is also “forcing a susceptible crop into a much broader area than we’ve ever done before. That’s especially true this year, from what I’m hearing with cotton ground being put in soybeans. One of the things I’m sure we’ll find is that root-knot nematodes have been managed to some degree with Temik in cotton. Put soybeans on that acreage and you’ll find where the root-knot nematodes are. Actually, rotation with either cotton or corn will give the same result because both of those crops increase root-knot nematode populations.”

Reniform nematodes are also in the Arkansas mix. Twenty-five years ago, “we knew of reniform nematodes in a couple of Jefferson County cotton fields and one in Monroe County. Today, we have reniform nematodes confirmed all the way from the Missouri Bootheel to the Louisiana line. They’re up and down the Mississippi River.”

Reniform has always been a cotton issue because “they go wild on that crop. But soybeans host them too. Frankly, we don’t know how bad reniform nematodes are on soybeans. We know they spread very rapidly across a field. From the point of introduction, in three or four years it’ll be field-wide.”

There are some reniform-resistant cultivars, but not many.

Races

Back in 1980s, an Arkansas farmland survey showed almost 70 percent of the soybean acreage in the state had soybean cyst nematodes. At that time, Race 3 was found almost exclusively. Resistant varieties — first to Race 3, then to Race 9, then Race 14 — worked very well. But Kirkpatrick said they were overused.

Fast-forward to this decade and surveys conducted in 2001-02 found races 2, 5 and 6 — with a small percentage of races 9 and 14 identified — but not Race 3.

“The pattern we’re seeing now is with Race 2, Race 5 and Race 6. We never find Race 3 and seldom find a Race 9 or 14.

“Lest you say, ‘that’s got to be a fluke,’ I looked back over the last couple of years and was struck by several things.

“First, soybean farmers aren’t sampling. Only 7 percent of the 3,000-plus samples that come to my lab are from soybean fields. Cotton farmers are doing a much more vigilant job of seeing if they’ve got nematodes in their field.

“Second, of the samples sent in from soybeans in 2006 and 2007, 63 percent had soybean cyst in 2006 and 66 percent had soybean cyst in 2007 — not much of a departure from the infested acreage figures from 20 years ago.”

A random survey in Jackson County — recently pulled from 25 fields county-wide — showed half had soybean cyst nematodes. “We haven’t run all the tests yet, but the ones we have done showed Race 2. The soybean cyst nematode is still alive and well.”

Root-knot is also alive and well and “can hurt you just a little or can wipe out entire pieces of your field. If you want a resistant variety against root-knot, you’ll have to grow a late Group 5 or early Group 6, maturities that have gone out of favor among Arkansas growers. There hasn’t been any resistance of significance in any earlier-maturing varieties.”

This year, in a normal field that came out of cotton and had root-knot — “but not so harsh we knew it would kill the crop” — Kirkpatrick and colleagues applied Telone II, a soil fumigant nematicide, in strips in the field. Telone II is an older product that only kills nematodes.

“It’s very effective and also very expensive and hard to apply. But it does provide a tool to help measure how much yield loss we’re getting. (It turns out) we were leaving about 10 bushels of yield in the field without controlling nematodes. That’s probably true if the nematodes are cyst or root-knot.”

What about reniform? Some literature says it can cause a third of the crop to be lost. Kirkpatrick isn’t sure he agrees, “but the only data we’ve got was gathered a few years ago by accident.

“I had a cotton test on a grower’s farm in Monroe County. After the first year, he decided to stop growing cotton. I asked, ‘What can we do with this field that’s loaded with reniform?’ So the grower went in and alternated 16-row strips of one of the few reniform-resistant varieties with a susceptible variety that was similar in maturity and yield.”

Once again, the resistant variety had a 10-bushel yield advantage.

“I’m not sure 10 bushels is the magic number, but I think that’s the low end of what nematodes are costing growers.”

When soybeans were $4 per bushel, no one wanted to spend the time, money or effort looking at nematode control or nematicides. Now, “we’re behind the 8-ball because there isn’t an existing database to draw from. We don’t know because no one has looked — and it isn’t just Arkansas, that’s across the South. We’ve got some catching up to do. Breeders need to get us some effective resistance. We’ve got to have something to work with.”

Researchers need to determine whether reniform is “really a problem, and how big a problem it is. I’ve got to tell you, 14 of our best soybean-growing counties have widespread reniform. It isn’t a small list and we don’t know how to deal with it.”

What about nematicides? Other than data gathered last year by Kirkpatrick and Scott Monfort, another state Extension plant pathologist, “none has been generated in the last 30 years in Arkansas. We must figure out how to supplement genetic resistance as it comes along until highly resistant, early-maturing cultivars are commercially available for our growers. Will nematicides capture back most of the lost 10 bushels in the absence of resistance? We just don’t know.”

The lengthy list of questions researchers are attempting to answer includes:

• What about Vydate? “It isn’t labeled for soybean but is used on cotton as a foliar application to supplement nematode control. Maybe that’s an option.”

• What about seed treatments? “Those are easy to use but are they going to provide any help? Will they provide help if combined with some of the moderate levels of resistance soybean breeders will release?”

• What about rotation? “Will we be able to find a rotation sequence that will help and is economically feasible? More importantly, can we avoid rotations that will cause an even bigger problem?”

The South, said Kirkpatrick, is the region “that deals most frequently with nematode problems and with an array of nematodes. We’re very limited on research information on how to effectively and economically manage them in soybeans. But work has started to answer some of our questions.”